It means the effective capacitance across the crystal. For the case you mention where you want a 30pf load, then you need to remember that the two capacitor to ground are effectively in series. To get a 30pf load would require a 60pf cap to ground from each leg.
Of course you also need to consider the capacitance provided by the rest of the circuit and adjust accordingly. More capacitance will pull the frequency slightly higher.
---Gary